For a multiday time frame, the built-in BOP is much like the built-in MS in that the calculation is on daily bars, not multiday bars. If one wants either calculated on multiday bars, he can plot it as a Custom Indicator.

Generally, I find BOP on multiday time frames not realizable in the sense that it uses future data to calculate its value.
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For a 5-Day time frame, the built-in BOP value for a bar is:

1) for bars not at the right-edge, the BOP value for the Daily bar three market days after the 5-Day bar date.

2) for the bar at the right-edge, the BOP value for the Daily bar with the 5-Day bar date.

This explains your observations, but not why one would construct such an indicator.